The dihydric phenols have achieved significant success in their commercial applications. Dihydric phenols are useful in the commercial manufacture of various polymers including the polyarylates, polyamides, epoxies, polyetherimides, polysulfones and the polycarbonates. Significant attention has been directed to the commercial preparations of the dihydric phenols. For many years it has been well known that the acid catalyzed reaction of phenol with specific aldehyde or ketone could prepare the 4,4'-dihydric phenol with specific groups derived from the aldehyde or the ketone connecting the two phenolic rings. In particular when phenol is reacted with acetone, the dihydric phenol 4,4'(dihydroxyphenyl)propane-2, hereafter known as bisphenol-A is formed. This has particular utility in polycarbonates, polyarylates and copolyestercarbonates as well as epoxies. In order to make certain polymers, in particular the polycarbonates, the bisphenol-A must be particularly pure, for example, as measured by color. Additionally, the process should be particularly efficient since the dihydric phenol costs contribute substantially to the cost of the final polymer. Therefore much attention has been directed to the recovery of bisphenol-A after preparation. Not only is recovery from the major stream containing large quantities of bisphenol-A important, but because of the economics involved, various side streams or "purge streams" also containing significant quantities of bisphenol-A should also be investigated for improved recovery techniques.
However, in the downstream processing of the composition prepared from the acid catalyzed condensation reaction, a loss in the quantity of desired dihydric phenol and a reduction in the color quality of the dihydric phenol composition has been observed when mixtures of the desired dihydric phenol, phenol and isomers of the desired dihydric phenol are separated in conventional distillation trains to recover the desired dihydric phenol. Generally, without an additive, portions of these materials readily degrade to a very dark color and undergo substantial chemical degradation. Significant reductions of the desired dihydric phenol, usually bisphenol-A, are observed.
It has now been found that this degradation of desired dihydric phenol as well as the color formation can be substantially inhibited by the addition of a phosphite or phosphonite to the composition comprising dihydric phenol, phenol and isomers of the dihydric phenol prior to the distillation step.